Optimizing the NA for guided wave laser devices (oscillators and amplifiers) provides several advantages. First, because the optical pump source and signal beams must be guided within the core of the guided wave laser device, the NA must be large enough to accommodate both sets of beams within the core without loss. Typically, a pump source will emit optical energy into the laser active core of a waveguide structure. In response, ions in the core will fluoresce and emit another optical signal. At the same time, an oscillator or other device will emit a signal beam into the core of the waveguide structure. The signal beam is then amplified by the optical signal resulting from the fluorescing ions in the core. To maximize amplification, the wavelength of the signal beam is preferably matched to the wavelength of the optical signal resulting from the fluorescing ions. However, the NA must not be too large, otherwise more amplified spontaneous emission (ASE) is trapped and the likelihood of parasitic oscillations increases. By allowing the maximum amount of ASE to escape from the core (i.e., those ASE rays beyond the solid angle defined by the optimized NA), a relatively higher degree of gain is attainable. This is especially important for high-gain amplifier devices, in that a higher gain is sustainable than would otherwise be the case if non-optimized cladding or air were used to surround the core.
It is also important to address the ASE rays that do enter the cladding, and implement ways to prevent them from reentering the core region, thereby further enhancing the gain within the core. These ways include (1) adding appropriate dopant(s) within the cladding layers (ideally the same dopant as for the optimized NA) to absorb the ASE within the bulk of the cladding, (2) fine grinding the outer surface of the cladding to scatter ASE, (3) adding an absorptive layer on the outer surface of the cladding, and (4) incorporating scattering sites within the cladding host itself.
The prior art for optimizing the NA of guided wave composite solid-state laser devices generally comprises two cases, where, in both cases, the cladding is assumed to be diffusion bondable to the core in question. For the two cases of prior art, either the NA is uniquely determined by a given undoped cladding material, or non-lasing dopants are added to the core itself to increase the core's refractive index and thereby alter the NA.
In the first case, these claddings are typically either composed of the undoped core material or an undoped medium of a similar chemical composition so that a good quality diffusion bond is assured. For the undoped core material as the cladding layer, the general supposition is that the refractive index will decrease when the laser active ion is removed from the crystalline medium to be replaced by the stoichiometric ion of the host. However, in many cases the resulting NA is too small to support either the pump or the signal or both because the change in refractive index for the undoped medium is quite small (nominally 10−3 to 10−4 per at. %), given that the typical doping level for a laser medium is on the order of 1 at. %. Hence, the difference in refractive index between the doped and undoped material is on the order of 10−3 to 10−4 which is usually not large enough to provide an adequate NA for guiding.
For the specific case of a 1% Yb:YAG core, the NA is too small when undoped YAG is utilized as the cladding medium. Of course, the actual NA will depend on the initial doping level of the laser active core in question. As a result, one disadvantage of using undoped cladding material of the same stoichiometry as the core is that the NA is generally too small to be useful. Another disadvantage of the undoped cladding approach is that even if the NA is appropriate for a given laser architecture, it is a point design. In the event that the parameters of the pump or signal beams change, or the ASE suppression needs to be enhanced, the undoped cladding material may not be able to address these issues. Yet another disadvantage results when the undoped cladding material comprises a different material than the core. In such a situation, the NA may be too large, and is completely dependent on whether a fortuitous match is found between a given core and cladding. In that case, the ASE and parasitic oscillations may degrade the laser performance and even lead to optical damage of the guided wave composite.
For the other case in which a non-lasing dopant is added to the core, it is preferable that the additional dopant or dopants not affect the laser performance in terms of introducing absorption at the pump or signal wavelength. It is important to note the distinction between the deleterious effect of absorption by such additional dopant(s) in the core, versus the beneficial advantage of that same absorption by the same dopant(s) in the cladding that suppresses ASE. Another constraint is the ability to introduce a dopant(s) in the crystal growth process for the core without negatively affecting the growth of the active laser medium.